Blender 2.8 Leaf Transparency Issue, with Eevee
There are (at least) 2 reasons why they are not transparent:


your leaf texture is a .jpg file: this format can't carry transparency informations. Only .png images can have a transparent background in the image itself.
You are connecting the Color output of the Image Texture node into the Fac of the Mix Shader, but the latter needs a black and white image, where black means transparent and white means opaque. You should use the Alpha output instead, which takes the transparency of your .png texture and transforms it into a BW map for the Mix Shader